I am having trouble collecting my first troll mutagen in Witcher 3. I have found several trolls in the game, but none of them have dropped a single troll mutagen. I have received advise from others to re-kill the trolls by saving the game and reloading it. I have tried this several times, and none of my trolls re-spawned. In fact, when I revisit locations where trolls were slain, they never reappear. I am unsure if this is a bug in the pc version or something else.
What must I do to force troll respawn in Witcher 3?